296 Dispersion of Fresh-water Fishes 



ment. Throughout all groups of animals and plants, there are 

 large branches similarly affected by peculiarities of conditions. 



This is the basis of the law of "x\daptive Radiation." Prof. 

 H. F. Osborn thus states this law: 



"It is a well-known principle of zoological evolution that an 

 isolated region, if large and sufficiently varied in its topography, 

 soil, climate, and vegetation, will give rise to a diversified fauna 

 according to the law of adaptive radiation from primitive and 

 central types. Branches will spring off in all directions to take 

 advantage of every possible opportunity of securing food. The 

 modifications which animals undergo in this adaptive radiation 

 are largely of mechanical nature; they are limited in number 

 and kind by hereditary stirp or germinal influences, and thus 

 result in the independent evolution of similar types in widely 

 separated regions under the law of parallelism or homoplasy." 



